The invention relates to a viscous coupling for lockingly connecting two parts which are rotatable relative to one another, having the following characteristics: a first coupling part is provided in the form of a rotational housing; a second coupling part is provided in the form of a hub; the first coupling part and the second coupling part are arranged so as to be rotatable relative to one another and, while being positioned co-axially inside one another, together form a sealed annular chamber; inside the chamber, there are axially arranged alternating first coupling plates in the shape of annular discs with means for providing a non-rotating connection with the first coupling part, and second coupling plates in the shape of annular discs with means for providing a non-rotating connection with the second coupling part; otherwise, the annular chamber is at least partially filled with a viscous fluid.
With couplings of said type, the shear forces in the viscous fluid cause a torque to be transmitted between the rotational housing and the hub, which shear forces occur between the first (outer) coupling plates and the second (inner) coupling plates, with the coupling plates being arranged parallel relative to one another and rotating relative to one another. As a rule, the viscous fluid is a high-viscosity silicone oil. The ability of the coupling to transfer torque is thus dependent on the existence of a relative speed between the rotational housing and the hub.
The use of viscous couplings of the present type is mainly based on the visco-transmission concept in motor vehicles wherein a first vehicle axle is permanently driven by the engine-gearbox unit and a second driving axle is driven by an auxiliary output by the engine gearbox unit via a viscous coupling of the present type. The operating principle of such drives consists in that when the driving axles rotate at identical speeds, i.e., under undisturbed driving conditions, only the first axle is driven, because under such conditions, the viscous coupling cannot transmit any torque. When slip occurs at the first driving axle, i.e. when the speed of the first axle does not correspond to the driving speed, the difference in speeds causes a transferable torque to be built up in the viscous coupling. Consequently, if traction is lacking at the first axle, torque is transmitted to the second axle to generate adequate driving forces. This behavior is not unproblematical under all circumstances, and in particular, it is not always compatible with the effective use of anti-lock braking systems and driving dynamics control systems.
In DE 38 34 555 C1 it has already been proposed to completely disconnect a viscous couplingxe2x80x94regardless of a relative speed between the housing and hubxe2x80x94in that the plates of the one coupling part are uncoupled from their coupling part, whereupon they are driven by the plates of the other coupling part at the speed of the plates of the other coupling part. A transmission of torque from the one coupling part to the other coupling part is thus eliminated, and the outer plates connected to the housing are coupled and separated in that annular coupling discs are inserted between said plates and that the outer plates are moved into a friction locking position relative to the coupling discs by being axially pressed together.
DE 40 36 230 C2 describes a viscous coupling of substantially the same type, but the inner plates, in principle, are freely rotatable relative to the hub and arranged so as to alternate with annular coupling discs which are held on the hub in a non-rotatable and axially movable way. By axially pressing together the inner plates and the coupling discs, they are moved into a friction-locking condition, so that the speed of the inner plates can be coupled to, or uncoupled from, that of the hub. Uncoupling can also be effected in response to different slip conditions.
It is the object of the present invention to provide an apparatus and method for effecting viscous coupling of the above-mentioned type which comprises control mechanisms for improving adaptation to different driving conditions. The objective is achieved in that the coupling plates, in their entirety, are divided into at least two functionally different sets of coupling plates each of which set comprises first (outer) and second (inner) coupling plates and forms separated axial regions positioned one behind the other and that at least one set of coupling plates is provided with means for allowing the first (outer) or the second (inner) coupling plates to be controllably connected to, or separated from, the respective coupling part. According to a first embodiment, it is proposed that a first set of first and second coupling plates is non-rotatably connected to the respective coupling part and that the second set of first and second coupling plates comprises means allowing the first or the second coupling plates to be controllably coupled to, and separated from, the respective coupling part. According to a further embodiment it is proposed that two sets of first and second coupling plates comprise means allowing the first or the second coupling plates of said sets, in a relatively stepped and controllable way, to be coupled to, and separated from, the respective coupling part. According to yet a further embodiment it is proposed that a first set of first and second coupling plates is non-rotatably connected to the respective coupling part and that two sets of first and second coupling plates comprise means allowing the first or the second coupling plates of said sets, in a relatively stepped and controllable way, to be coupled to, and separated from, the respective coupling part. The solution in accordance with the present invention differs from the attempt to control the coupling characteristics entirely via different slip values when coupling the coupling plates according to different characteristic curves; it differs in that it provides a simpler and less sensitively controllable device in which different preferred characteristic curves are predetermined by the design. To the extent that only said different characteristic curves are to be used for controlling purposes, the coupling and separating means can be simplified considerably because they can provide only two or three switching conditions to operate with two or three different characteristic curves. However, in principle, it is also possible to operate with different characteristic curves by setting a permanent slip value.
If, according to the first or the third embodiment, it is proposed that a first set of coupling plates is provided with first and second coupling plates which are permanently coupled to the housing and hub respectively, it is ensured that the coupling is permanently set to transfer a small torque, thus achieving the desired damping effect in the driveline, as a result of which backlash symptoms in the driveline can be damped, i.e. any play-related changes in contact in the driveline, especially in universal joints, are less violently transmitted to the engine-gearbox unit. These characteristics with only a slight locking effect are preferably set for maneuvering purposes, with detection taking place via a certain minimum steering angle, when using the anti-lock braking system for braking purposes and the driving dynamics control system.
On the other hand, the highest possible locking effect has to be set when starting the vehicle (from the stationary condition, first gear), when operating with an off-highway reduction stage (if available) and when reversing.
If the above-mentioned preferred embodiment comprises a total of three functionally different sets of coupling plates, it is proposed that, at an increased driving speed, e.g., at a speed in excess of 80 km/h, use is made of a mean characteristic curve, i.e., that one set of the two connectable sets is connected in addition to the permanently engaged set of coupling plates. As compared to maximum connection, this leads to a reduction in fuel consumption.
As already mentioned in connection with the above-referenced state of the art, in this case, too, the preferred means for controllably coupling and separating first or second coupling plates rotatably arranged in the chamber and provided for coupling and separating purposes are provided in the form of coupling discs which are positioned between said coupling plates, which are non-rotatably connected to the respective coupling part and are axially movable in the chamber, as well as setting means for axially displacing the coupling plates and the coupling discs against an abutment. If there are provided two functionally different controllable sets of coupling plates, it is proposed that the means for carrying out controllable coupling and separating operations act directly on the one set of coupling plates which is supported on a resiliently supported abutment which, in turn, acts directly on the second set of coupling plates which are supported on an axially fixed abutment. In such a case, as the setting forces and the setting distance increase, first the one set is connected and then the other set wherein the spring force of the abutment must be suitable for moving the first set of coupling plates into a position of complete friction locking relative to the coupling discs.
As is also known from the state of the art, a setting device for actuating means for carrying out controllable coupling and separating operations can comprise two cam discs which are rotatable relative to one another and of which one is axially fixed and the other one axially movable relative to the housing; the axially fixed cam disc can be rotatably drivable and the axially displaceable cam disc in non-rotatable and can act via a pressure bearing on a pressure plate which rotates together with the housing and which forms part of the means for controllably carrying out coupling and separating operations. For the setting device, there is preferably provided an electric drive: to achieve rapid reaction times, both the coupling operation and the separating operation are carried out electrically, so that the reaction speed is only negligibly dependent on the returning force of said springs. Stationary conditions are maintained electrically by providing constant voltage. There is no need for a holding brake. Only when the electric motor is set currentless, return is effected by the spring force, so that the coupling plates are separated from the coupling discs and uncoupled from the housing.